1. Field of the Invention
This invention relates to the charging of direct current storage batteries.
2. Description of the Prior Art
Battery charging systems have been devised for automatically recharging batteries while protecting the system against damage from high current flow. The charging current is often regulated in such systems through some type of phase control circuit for varying the portion of the AC cycle during which charging occurs, according to the existing charge on the battery.
There has heretofore not been known an automatic battery charging system wherein effective current limiting circuitry is provided for protecting the system, particularly against high currents experienced when the battery is severely discharged and hence is calling for a high charging rate. In most automatic battery chargers, the charging rate is regulated according to the current in the DC charger output circuit or in the primary circuit of the AC transformer; such circuits are inadequate in that they only operate properly until the battery voltage drops about 20% of its normal charge, and then either cease to function or else the charger is shut off and on out of synchronization until a circuit breaker is tripped or a fuse blows to save the circuit.
Additionally, present automatic battery chargers lack circuitry for compensating effectively for leakage from a charged battery. Some automatic battery chargers have controlled rectifiers for transmitting current to the battery being charged according to signals generated from a phase control circuit. The latter circuit generally employs a capacitive device which is charged and discharged according to the voltage in the battery. However, when the battery is fully charged, a small amount of leakage will occur and the capacitive device can charge and discharge randomly--out of synchronization with the AC source, wherefore the leakage is not compensated for effectively. This phenomenon is known as "latching". Prior attempts to correct "latching" have involved the use of extra, usually costly, circuitry.
Another shortcoming of existing battery chargers is their failure to equalize the charge in the respective cells of the battery. It is known that the cells of a battery have an ideal voltage and that these cells tend to discharge or leak at varying rates, causing their voltages to vary. For instance, the ideal voltage for a 6 cell, 12 volt battery is 2.17 volts per cell (or 13.02 volts for the 12 volt battery) under which conditions the battery has a maximum life. As a result of the variation in cell voltage, the more highly charged cells generate more gas and use more water. Therefore battery manufacturers recommend that batteries be slightly overcharged to equalize the charge on each cell.
U.S. Pat. No. 3,296,515 (Knauth) discloses a battery charger for charging at a rate commensurate with the charge below normal of the battery. Protection is provided against damage from short circuits, reverse polarity hook-ups, and other deleterious situations. The system includes a controlled power rectifier which is connected to the battery, and which is rendered conductive by a gate firing circuit according to the voltage difference between the rated voltage of a zener diode and the actual battery voltage. There is no compensation for leakage once the battery is charged. A pulsating DC signal rather than pure pulses, controls the gating signal to the lower rectifier, and as the battery approaches a full charge the gating signals become erratic, causing the rectifier to fire randomly. Also, since the charger turns off once the battery is charged, a switch is provided to reactivate the charger.
A prior patent to the present inventor, U.S. Pat. No. 3,586,955, discloses a battery charger wherein a transformer is coupled to a silicon controlled rectifier (SCR) whose power output is controlled by a sensing network coupled to a phase control circuit. The firing of the SCR is effected by pulses generated by a unijunction transistor. Circuit protection is provided for limiting the current in the system by leaking some of the charge from the capacitor in the unijunction circuit to delay the firing of the SCR when the load on the battery exceeds the charging rate of the charger. Although this was an effective charger, its effectiveness diminished for severely discharged batteries, and there was no compensation for leakage from a charged battery. Also, no short circuit or reverse polarity battery protection was provided.
A battery charger employing a pair of SCRs triggered by unijunction circuits is the subject of U.S. Pat. No. 3,766,463 (Ruben). The unijunction circuits are fired under the control of pedestal and cosine modified ramp trigger circuits. A special circuit is provided for preventing the unijunction circuit from being latched on. Although some system protection devices are incorporated in the charger, there is no provision for limiting the current therein or for reverse battery polarity connection.
U.S. Pat. No. 3,930,197 (Saylor) discloses a synchronous voltage regulator system having a permanent magnet coupled to three phase windings. Current output is regulated by SCRs which are gated by a ramp and pedestal unijunction circuit. If excessive currents occur the unijunction will latch on and quit, wherefor a special (and expensive) four-layer diode is provided to bypass the excess current.
The prior art fails to disclose a battery charger which effectively charges a severely discharged battery, smoothly with perfect synchronization between the charging current and the phase control circuit; prior chargers fail to compensate for leakage from charged batteries; and prior chargers fail to equalize the voltage between the cells of a battery. The prior art suffers from other deficiencies which are not present in the invention described herein.